Controlling Factors and Phase State of Hydrocarbons in the Upper Ordovician of the Tazhong Area,Tarim Basin

CHEN Jun-qing;PANG Xiong-qi;JIANG Zhen-xue;PANG Hong;State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum;Basin & Reservoir Research Center,China University of Petroleum;Unconventional Natural Gas Research Institute,China University of Petroleum;

There have been seven hydrocarbon reservoirs discovered so far in the Upper Ordovician in Tazhong area,of which the hydrocarbon phase distribution is complicated.The distribution features and controlling factors of the hydrocarbon phase in the Tazhong area were investigated based on the geological and geochemical conditions for formulation and distribution of hydrocarbon reservoirs.The results indicated that hydrocarbon reservoir types in the Upper Ordovician of Tazhong area,from west to east,varied from unsaturated to saturated condensate gas reservoirs;and from north to south,changed from condensate gas reservoir to unsaturated oil reservoirs.The regular sterane of the crude oil is of both the "reverse-L type"C27≤C28C29 which means from Lower-Middle Cambrian and the V-type regular sterane C27C28C29 which is from Middle-Upper Ordovician source rocks,so that the crude oil mainly came from the mixture of the Lower-Middle Cambrian and Middle-Upper Ordovician source rocks,while the drying coefficients and carbon isotopes suggested that the natural gas had entered the high to overmature stage of evolution,failing to match the mature stage of the Middle-Upper Ordovician source rock,showing that the natural gas mainly from the cracking gas of Lower-Middle Cambrian crude oil.The carbon isotopes of natural gas are mainly light,and the relationship between Ln(C2/C3)and Ln(C1/C2)exhibits a positive correlation,thus,the gas was considered to be crude oil cracking gas.The hydrocarbon phase distribution was mainly controlled by temperature,pressure,multiple periods of hydrocarbon accumulation and alteration.The temperature and pressure of the hydrocarbon reservoirs were low in early stage,then they underwent two periods of tectonic uplifts,which altered the critical temperature(Tc)and the critical condensate temperature(Tm)of the early reservoirs,forming unsaturated oil reservoirs.With the increase of burial depth in late stage,the temperature and pressure of reservoirs became high due to the oil-cracking gas flushing of the early oil,forming unsaturated condensate-gas reservoirs.The Lower-Middle Cambrian reservoir-cap assemblage of high quality may be an important target for the future exploration of natural gas in Tazhong area.